Q: Every year, my water lilies and glads make it through the winter just fine in the basement, but my cannas and dahlias never survive. I leave the water lilies in their pots; the other tubers and corms I store in mesh bags. What am I doing wrong? Are other summer bulbs less finicky? — Linda Whifield, Boulder, Colo.
A: If you have a spot where the temperature stays between 40 and 55 degrees all winter, it will be easy to keep almost any kind of summer bulb going. Depending on your climate, a garage or an attic, an unheated spare room or a basement might do. Neither cannas nor dahlias are especially finicky, but you can’t treat them like so many onions or potatoes. Unlike your glads, they need a bit of moisture: they’ll dry out in mesh bags, especially in buildings with central heating. They need to be on the dry side but not superdry. And don’t store them in closed containers; without air circulation they’ll rot.
One common method is to put them in a box lined with plastic or in a trash bag that’s left loosely open. Add a little damp sawdust or peat (a gallon or two per trash bag); then set the container in a cool, dark room. Check the container monthly; if the roots and sawdust get too dry, give the bag a spritz or two of water. Just ever so slightly moist is ideal. Leave the top open, and don’t stack the containers or you’ll block the airflow. There are a few more tricks. Clusters of dahlia tubers are best kept intact over winter because the eyes (little buds, as on a potato) that will produce the next year’s growth are very near the main stem and hard to see when you dig. If you separate a tuber and it doesn’t have an eye, nothing will grow; any breaks in the skin are prone to rot. By late winter the eyes will be very obvious; at that point you may cut the tubers apart, using a sharp knife or pruning clippers. The only exception to this rule applies to the tired old parent tuber, which is under the group of new ones. Remove it, and hose off the cluster; let it dry for an hour or two; then put it in your container. For dahlias, the ideal temperature is about 45 degrees, but they’ll be fine as long as they never freeze. Next spring, when you are dividing your dahlias for planting, you’ll benefit from knowing that bigger is not better here; small and medium-size dahlia tubers produce better plants than the largest tubers do.
Cannas, on the other hand, need things a bit warmer. Keep temperatures above 40 degrees (up to about 60 degrees is acceptable, although you may need to spritz them more often). Another difference: Divide canna rhizomes when you dig them, which should be after the first hard frost. If you cut or break up the sections so that each has three to five strong eyes, they will pack much better in your storage containers. Many smaller, single- and double-eyed pieces will break off in this process. Don’t be alarmed; just pack them, too. Next spring, plant three to five of these single pieces in each hole, and you’ll get a canna as impressive as one from a hefty division.
The other popular summer bulbs — tuberous begonia, caladium, and elephant’s-ear (colocasia) — should be stored very dry, like glads (an open box or a mesh bag like yours provides ventilation, and you may dispense with sawdust). But each of these also has quirks. Glads should be dug much earlier, about two weeks after flowering or as soon as the foliage shows the first signs of yellowing. Tuberous begonias will flower profusely from midsummer and well into autumn. But when frost blackens the foliage, dig up the tubers, shake off most of the soil, and let them dry indoors for a week. Then clean them up, and put them in a cool, dark place where they’ll stay dry. Although temperatures between 35 and 80 degrees are fine, the extra warmth will cause them to sprout early, in some places even in January, whether or not you’re ready. If you pot them right away, they’ll be all right. For northern gardeners, cooler is better with begonias.
Is all this effort worth it? Do you get a better display from the summer bulbs you save, or are those you buy just as good? With dahlias and cannas the advantage is mostly economic. Purchased roots will perform about as well as the ones you save yourself. But for the others, the roots you dig in the fall will be a little bigger than the ones you planted in the spring. For glads the advantage is good for only a year; after its third season a glad will go into decline and will peter out totally after about five years. (You can grow new ones from the little cormels that you’ll find when you dig; they take one season to grow to blooming size.) Tuberous begonias gradually get better each year for about eight years, and then they die. To get new ones, you’ll have to root cuttings, start a new plant from seed, or just buy new tubers. Size matters most of all with colocasias and caladiums. Those roots live a long time and gradually get bigger, producing more (though not necessarily larger) leaves with each passing season.